linux-kernel - Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

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Message-ID: <c9d89644-409e-0363-69f0-a3b8f2ef0ae4@collabora.com>
Date:   Mon, 15 Aug 2022 17:57:30 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Christian König <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>, Huang Rui <ray.huang@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        Trigger Huang <Trigger.Huang@...il.com>,
        Gert Wollny <gert.wollny@...labora.com>,
        Antonio Caggiano <antonio.caggiano@...labora.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>, kvm@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

On 8/15/22 16:53, Christian König wrote:
> Am 15.08.22 um 15:45 schrieb Dmitry Osipenko:
>> [SNIP]
>>> Well that comment sounds like KVM is doing the right thing, so I'm
>>> wondering what exactly is going on here.
>> KVM actually doesn't hold the page reference, it takes the temporal
>> reference during page fault and then drops the reference once page is
>> mapped, IIUC. Is it still illegal for TTM? Or there is a possibility for
>> a race condition here?
>>
> 
> Well the question is why does KVM grab the page reference in the first
> place?
> 
> If that is to prevent the mapping from changing then yes that's illegal
> and won't work. It can always happen that you grab the address, solve
> the fault and then immediately fault again because the address you just
> grabbed is invalidated.
> 
> If it's for some other reason than we should probably investigate if we
> shouldn't stop doing this.

CC: +Paolo Bonzini who introduced this code

commit add6a0cd1c5ba51b201e1361b05a5df817083618
Author: Paolo Bonzini <pbonzini@...hat.com>
Date:   Tue Jun 7 17:51:18 2016 +0200

    KVM: MMU: try to fix up page faults before giving up

    The vGPU folks would like to trap the first access to a BAR by setting
    vm_ops on the VMAs produced by mmap-ing a VFIO device.  The fault
handler
    then can use remap_pfn_range to place some non-reserved pages in the
VMA.

    This kind of VM_PFNMAP mapping is not handled by KVM, but follow_pfn
    and fixup_user_fault together help supporting it.  The patch also
supports
    VM_MIXEDMAP vmas where the pfns are not reserved and thus subject to
    reference counting.

@Paolo,
https://lore.kernel.org/dri-devel/73e5ed8d-0d25-7d44-8fa2-e1d61b1f5a04@amd.com/T/#m7647ce5f8c4749599d2c6bc15a2b45f8d8cf8154

-- 
Best regards,
Dmitry